Substernal leadless electrical stimulation system

ABSTRACT

Implantable leadless cardiac pacing systems and methods for providing substernal pacing using the leadless cardiac pacing systems are described. In one embodiment, an implantable leadless cardiac pacing system includes a housing, a first electrode on the housing, a second electrode on the housing, and a pulse generator within the housing and electrically coupled to the first electrode and the second electrode. The housing is implanted substantially within an anterior mediastinum of a patient and the pulse generator is configured to deliver pacing pulses to a heart of the patient via a therapy vector formed between the first and second electrodes.

This application claims the benefit of U.S. Provisional Application No.61/820,033, filed on May 6, 2013, the content of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present application relates to leadless electrical stimulationdevices, systems and methods for providing substernal electricalstimulation, including substernal cardiac pacing.

BACKGROUND OF THE INVENTION

Implantable pulse generators have been utilized to provide electricalstimulation to various organs, tissues, muscle, nerves or other featuresof a patient's body. One example of electrical stimulation provided to apatient is cardiac pacing. Cardiac pacing electrically stimulates theheart when the heart's natural pacemaker and/or conduction system failsto provide synchronized atrial and ventricular contractions atappropriate rates and intervals for a patient's needs. When a patient'sheart is beating too slow, bradycardia pacing increases the rate atwhich the patient's heart contracts to provide relief from symptomsassociated with bradycardia. Cardiac pacing may also provide electricaloverdrive stimulation intended to suppress or convert tachyarrhythmias,again supplying relief from symptoms and preventing or terminatingarrhythmias that could lead to sudden cardiac death or need to betreated with high voltage defibrillation or cardioversion shocks.

Pacemakers typically require at least two electrodes to deliverelectrical stimulation therapy to the heart and to sense electricalactivity of the heart. Traditionally, pacemaker systems are comprised ofan implantable pulse generator (or pacemaker) coupled to one or moreleads. The lead(s) include one or more electrodes on a distal portion ofthe lead that are implanted inside the heart such that at least oneelectrode touches the endocardium. In other examples, the one or moreleads can be implanted on the epicardial surface of the heart. In stillfurther examples, leadless pacing devices may be implanted within one ormore chambers of the heart and provide pacing pulses to the heart.

SUMMARY OF THE INVENTION

The present application is directed to implantable cardiac pacingsystems and methods for providing substernal pacing. In one embodiment,an implantable cardiac pacing system includes a housing, a firstelectrode on the housing, a second electrode on the housing, and a pulsegenerator within the housing and electrically coupled to the firstelectrode and the second electrode, wherein the housing is implantedsubstantially within an anterior mediastinum of a patient and the pulsegenerator is configured to deliver pacing pulses to a heart of thepatient via a therapy vector formed between the first and secondelectrodes.

In another embodiment, a method comprises providing a leadlessimplantable pulse generator (IPG) substantially within an anteriormediastinum of a patient, the leadless IPG including a housing, firstelectrode on the housing, a second electrode on the housing, and anpulse generator within the housing and electrically coupled to the firstelectrode and the second electrode, generating one or more stimulationpulses with the implantable pulse generator, and delivering the one ormore stimulation pulses to a heart of the patient via the first andsecond electrode of the leadless IPG implanted substantially within theanterior mediastinum.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front view of an example leadless IPG implanted within apatient.

FIG. 1B is a side view of an example leadless IPG implanted within apatient.

FIG. 1C is a transverse view of an example leadless IPG implanted withina patient.

FIG. 2 is a conceptual view illustrating the leadless IPG of FIG. 1.

FIG. 3A is a front view of another example leadless IPG implanted withina patient.

FIG. 3B is a side view of the leadless IPG of FIG. 3A implanted withinthe patient.

FIG. 4A is a front view of a further example leadless IPG implantedwithin a patient.

FIG. 4B is a side view of the leadless IPG of FIG. 4A implanted withinthe patient.

FIG. 5 is a functional block diagram of an example configuration ofelectronic components of an example leadless IPG.

FIG. 6 is a graph illustrating strength-duration curves showing thecapture thresholds obtained at various pulse widths during a first acutestudy.

FIG. 7 is a graph illustrating strength-duration curves showing thecapture thresholds obtained at various pulse widths during a secondacute study.

FIG. 8 is a graph illustrating strength-duration curves of electricaldata from a third acute experiment.

FIG. 9 is a graph illustrating strength-duration curves of electricaldata from the third acute experiment.

FIG. 10 is a graph illustrating strength-duration curves of electricaldata from a third acute experiment.

DETAILED DESCRIPTION

FIGS. 1A-C are conceptual diagrams of an example leadless implantablepulse generator (IPG) 10 implanted within a patient 12. FIG. 1A is afront view of patient 12 implanted with leadless IPG 10. FIG. 1B is aside view of patient 12 with leadless IPG 10. FIG. 1C is a transverseview of patient 12 with leadless IPG 10. FIGS. 1A-C are described in thecontext of implantable cardiac pacing. However, the techniques of thisdisclosure may also be used in the context of other implantable medicaldevices configured to provide electrical stimulation pulses to stimulateother organs, tissues, muscles, or nerves within the body of patient 12.For example, leadless IPG 10 implanted in the manner described hereinmay provide electrical stimulation pulses to stimulate nerves, skeletalmuscles, diaphragmatic muscles, e.g., for various neuro-cardiacapplications and/or for apnea or respiration therapy.

Leadless IPG 10 is implanted underneath/below sternum 22 substantiallywithin anterior mediastinum 28. Anterior mediastinum 28 may be viewed asbeing bounded laterally by pleurae 32, posteriorly by pericardium 30,and anteriorly by sternum 22. In some instances, the anterior wall ofanterior mediastinum 28 may also be formed by the transversus thoracisand one or more costal cartilages. Anterior mediastinum 28 includes aquantity of loose connective tissue (such as areolar tissue), some lymphvessels, lymph glands, substernal musculature (e.g., transverse thoracicmuscle), branches of the internal thoracic artery, and the internalthoracic vein. For example, leadless IPG 10 may be implantedsubstantially within the loose connective tissue and/or substernalmusculature of anterior mediastinum 28. A leadless IPG implantedsubstantially within anterior mediastinum 28 will be referred to hereinas a substernal leadless IPG. Also, electrical stimulation, such aspacing, provided by a leadless IPG implanted substantially withinanterior mediastinum 28 will be referred to herein as substernalelectrical stimulation or substernal pacing.

Although leadless IPG 10 is described herein as being implantedsubstantially within anterior mediastinum 28, leadless IPG 10 may beimplanted in other non-vascular, extra-pericardial locations, includingthe gap, tissue, or other anatomical features around the perimeter ofand adjacent to, but not attached to, the pericardium or other portionof heart 26 and not above sternum 22 or ribcage. As such, leadless IPG10 may be implanted anywhere within the “substernal space” defined bythe undersurface between the sternum and/or ribcage and the body cavitybut not including the pericardium or other portion of heart 26. Thesubsternal space may alternatively be referred to by the terms“retrosternal space” or “mediastinum” or “infrasternal” as is known tothose skilled in the art and includes the anterior mediastinum 28. Thesubsternal space may also include the anatomical region described inBaudoin, Y. P., et al., entitled “The superior epigastric artery doesnot pass through Larrey's space (trigonum sternocostale).” Surg. Radiol.Anat. 25.3-4 (2003): 259-62 as Larrey's space. In other words, leadlessIPG 10 may be implanted in the region around the outer surface of heart26, but not attached to heart 26.

Leadless IPG 10includes a housing 14 having electrodes 16 and 18.Leadless IPG 10 may be implanted substantially within anteriormediastinum 28 such that leadless IPG 10 can sense electrical activityof heart 26 and/or deliver electrical stimulation, e.g., pacing, toheart 26 via electrodes 16 and 18. In one example, leadless IPG 10 maybe implanted such that electrodes 16 and 18 are located substantiallyover a cardiac silhouette of one or both ventricles as observed via ananterior-posterior (AP) fluoroscopic view of heart 26. In anotherexample leadless IPG 10 may be implanted such that a bipolar therapyvector between electrodes 16 and 18 is centered or otherwise locatedover the ventricle(s). However, leadless IPG 10 may be positioned atother locations as long as the bipolar therapy vector between electrodes16 and 18 result in capture of the ventricle(s) of heart 26.

In the example illustrated in FIGS. 1A-C, leadless IPG 10 is locatedsubstantially centered under sternum 22. In other instances, however,leadless IPG 10 may be implanted such that it is offset laterally fromthe center of sternum 22. In some instances, leadless IPG 10 may extendlaterally enough such that all or a portion of leadless IPG 10 isunderneath/below the ribcage in addition to or instead of sternum 22.

Additionally, although leadless IPG 10 is described in the context ofproviding ventricular pacing, leadless IPG 10 may be placed to provideatrial pacing. In this case, leadless IPG 10 may be positioned furthersuperior within anterior mediastinum 28 such that leadless IPG 10 candeliver electrical stimulation, e.g., pacing, to an atrium of heart 26via electrodes 16 and 18. For example, leadless IPG 10 may be positionedwithin anterior mediastinum 28 such that electrodes 16 and 18 arelocated over the atrium as observed in an AP fluoroscopic view of heart26 and/or such that the bipolar therapy vector between electrodes 16 and18 is substantially over the atrium or otherwise capable of capturingthe atrium of heart 26.

In addition, it should be noted that leadless IPG 10 may not be limitedto treatment of a human patient. In alternative examples, leadless IPG10 may be implemented in non-human patients, e.g., primates, canines,equines, pigs, ovines, bovines and felines. These other animals mayundergo clinical or research therapies that may benefit from the subjectmatter of this disclosure.

FIG. 2 is a conceptual view illustrating leadless IPG 10 of FIG. 1. Asillustrated in FIG. 2, leadless IPG 10 includes housing 14, electrodes16 and 18, and a spacer 34. Housing 14 forms a hermetic seal thatprotects components of leadless IPG 10. As will be described in furtherdetail herein, housing 14 may protect one or more processors, memories,transmitters, receivers, sensors, sensing circuitry, therapy circuitry,power sources, and other appropriate components.

Housing 14 may take on any of a number of shapes. In the exampleillustrated in FIG. 2, housing 14 is generally cylindrical orpill-shaped. Housing 14 may have any of a number of shapes/dimensions.For example, housing 14 may be more of flat, rectangular shape. In oneexample, housing 14 may be less than approximately 30 mm in length andbe less than or equal to 20 French. In other examples, housing 14 may belarger than 30 mm in length such that electrodes on the housing may belocated over the atria and the ventricles.

Housing 14 of may be substantially formed of a conductive material, suchas a medical grade stainless steel, titanium alloy, or other metal ormetal alloy. Housing 14 also includes an insulative layer formed over atleast a portion of housing 14, such as a layer of parylene, polyimide,or urethane. In some examples, electrodes 16 and 18 may be defined byuninsulated portions of an outward facing portion of housing 14. In someinstances the insulative layer may be formed to focus, direct or pointthe electrodes toward heart 26. For example, the insulative layer maynot extend around the full circumference of housing 14 to form anelectrode 16 and/or 18 that only extends around a portion of thecircumference of housing 14 (e.g., half, quarter or other portion).Housing 14 also includes a non-conductive spacer 34 that separates theportion of housing forming electrode 16 from the portion of housingforming electrode 18. Other division between insulated and uninsulatedportions of housing 14 may be employed to define a different number orconfiguration of housing electrodes. In other instances, electrode 16and/or 18 may be otherwise coupled to housing 14.

Electrodes 16 and 18 are illustrated in FIG. 2 as ring or cylindricalelectrodes disposed on the exterior surface of housing 14. In oneexample, electrodes 16 and 18 may each have surface areas betweenapproximately 2-55 mm². In another example, one or both of electrodes 16and 18 may have surface areas up to 200 mm². Electrode 16 and electrode18 may have an electrode spacing of between approximately 5-50 mm.Electrode 16 may be used as a cathode and electrode 18 may be used as ananode, or vice versa, for delivering electrical stimulation therapy toand/or sensing electrical signals associated with heart 26. In otherexamples, electrode 16 and/or 18 may be formed in other shapes, such asa hemispherical electrode that includes one of the ends of housing 14 orthat does not extend around the entire circumference of housing 14.

Leadless IPG 10 may further include one or more anchoring mechanismsthat are positioned along the length of the housing 14. The anchoringmechanisms may affix leadless IPG 10 to the loose connective tissue orother structures of the anterior mediastinum 28 to reduce movement ofleadless IPG 10 from its desired location. The one or more anchoringmechanism(s) may either engage bone, fascia, muscle or other tissue ofpatient 12 or may simply be wedged therein to affix leadless IPG 10under sternum 22 to prevent excessive motion or dislodgment.

Leadless IPGs of this disclosure may take on various otherconfigurations. For example, a leadless IPG of this disclosure mayconform with the leadless IPG illustrated and described in FIG. 2 andparagraphs [0040]-[0045] of copending U.S. patent application entitled,“IMPLANTABLE MEDICAL DEVICE SYSTEM HAVING IMPLANTABLECARDIOVERTER-DEFIBRILLATOR (ICD) SYSTEM AND SUBSTERNAL LEADLESS PACINGDEVICE” (Attorney Docket No. C00005682.USU2) filed on the same day asthe current application. The entire content of the referenced portionsare incorporated herein by reference in their entirety.

FIGS. 3A and 3B are conceptual diagrams of another example leadless IPG40 implanted within patient 12. FIG. 3A is a front view of patient 12implanted with leadless IPG 40. FIG. 3B is a side view of patient 12with leadless IPG 40. Leadless IPG 40 conforms substantially toimplantable cardiac leadless IPG 10 of FIGS. 1A-1C, but housing 42 ofleadless IPG 40 electrodes 44 and 46 in addition to electrodes 16 and18. Repetitive description of like numbered elements described in otherembodiments is omitted for sake of brevity.

Housing 42 conforms substantially to housing 14 of leadless IPG 10.Thus, housing 42 may include the structure and/or function describedabove with respect to housing 14. Housing 42 may, however, have a lengththat is longer than housing 14 of leadless IPG 10 to enable a firstportion of housing 42 including electrodes 44 and 46 to be implantedover an atrium of heart 26 and a second portion of housing 42 includingelectrodes 16 and 18 to be implanted over a ventricle of heart 26 (e.g.,as viewed via an AP fluoroscopic view of heart 26).

Electrodes 44 and 46 conform substantially to electrodes 16 and 18.Therefore, description of electrodes 16 and 18 will not be repeatedhere, but is equally applicable to electrodes 44 and 46. Housing 42 mayalso include additional non-conductive spacers (not shown) to isolateelectrodes 44 and 46 from one another and to isolate electrodes 44 and46 from electrodes 16 and 18. In other embodiments, housing 42 mayinclude more or fewer electrodes. For example, housing 42 may includeonly two electrodes with the first electrode being placed near the atriaof the heart to sense and/or pace the atrium and the second electrodebeing placed near the ventricle of the heart to sense and/or pace theventricle. In this manner, leadless IPG 40 may provide multi-chambersensing and/or pacing.

FIGS. 4A and 4B are conceptual diagrams of an example cardiacstimulation system 50 that includes multiple leadless IPGs implantedwithin patient 12. FIG. 4A is a front view of patient 12 implanted withimplantable cardiac stimulation system 50. FIG. 4B is a side view ofpatient 12 with implantable cardiac stimulation system 50. Repetitivedescription of like numbered elements described in other embodiments isomitted for sake of brevity.

Cardiac stimulation system 50 includes a first leadless IPG 10 and asecond leadless IPG 52. Leadless IPG 10 is described above in detailwith respect to FIGS. 1A-1C. Leadless IPG 52 (e.g., housing 54 andelectrodes 56 and 58) conforms substantially to leadless IPG 10 (e.g.,housing 14 and electrodes 16 and 18). As such, the structure and/orfunction described above with respect to leadless IPG 10, housing 14,and electrodes 16 and 18 are equally applicable to leadless IPG 52,housing 54, and electrodes 56 and 58.

Leadless IPG 52 is implanted substantially within anterior mediastinum28 such that leadless IPG 52 can sense electrical activity of the atriaof heart 26 and/or deliver electrical stimulation to the atria of heart26 via electrodes 56 and 58. In one example, leadless IPG 52 may beimplanted such that electrodes 56 and 58 are located substantially overa cardiac silhouette of one or both atria as observed via an APfluoroscopic view of heart 26. In another example, leadless IPG 52 maybe implanted such that a bipolar therapy or sense vector betweenelectrodes 56 and 58 is centered or otherwise located over the atrium.However, leadless IPG 52 may be positioned at other locations as long asthe bipolar therapy vector between electrodes 56 and 58 result incapture of the atrium of heart 26. In this manner, cardiac stimulationsystem 50 may provide multi-chamber pacing. In another example, leadlessIPG 52 may be a sensing-only device.

In some instances, leadless IPG 10 and leadless IPG 52 may operateindependently of one another. In other instances, leadless IPG 10 andleadless IPG 52 may coordinate delivery of stimulation therapy bycommunicating with one another either via one-way or two-waycommunication.

FIG. 5 is a functional block diagram of an example configuration ofelectronic components of an example leadless IPG 10. However, thediscussion below is equally applicable to leadless IPG 40 and 52.Leadless IPG 10 includes a control module 60, sensing module 62, therapymodule 64, communication module 68, and memory 70. The electroniccomponents may receive power from a power source 66, which may be arechargeable or non-rechargeable battery. In other embodiments, leadlessIPG 10 may include more or fewer electronic components. The describedmodules may be implemented together on a common hardware component orseparately as discrete but interoperable hardware, firmware, or softwarecomponents. Depiction of different features as modules is intended tohighlight different functional aspects and does not necessarily implythat such modules must be realized by separate hardware, firmware, orsoftware components. Rather, functionality associated with one or moremodules may be performed by separate hardware, firmware, or softwarecomponents, or integrated within common or separate hardware or softwarecomponents.

Sensing module 62 is electrically coupled to electrodes 16 and 18 viaconductors internal to housing 14 of leadless IPG 10. Sensing module 62is configured to obtain signals sensed via one or more sensing vectorsformed by electrodes 16 and 18, and the housing electrode of leadlessIPG 10 and process the obtained signals.

The components of sensing module 62 may be analog components, digitalcomponents or a combination thereof. Sensing module 62 may, for example,include one or more sense amplifiers, filters, rectifiers, thresholddetectors, analog-to-digital converters (ADCs) or the like. Sensingmodule 62 may convert the sensed signals to digital form and provide thedigital signals to control module 60 for processing or analysis. Forexample, sensing module 62 may amplify signals from the sensingelectrodes and convert the amplified signals to multi-bit digitalsignals by an ADC. Sensing module 62 may also compare processed signalsto a threshold to detect the existence of atrial or ventriculardepolarizations (e.g., P- or R-waves) and indicate the existence of theatrial depolarization (e.g., P-waves) or ventricular depolarizations(e.g., R-waves) to control module 60.

Control module 60 may process the signals from sensing module 62 tomonitor electrical activity of heart 26 of patient 12. Control module 60may store signals obtained by sensing module 62 as well as any generatedEGM waveforms, marker channel data or other data derived based on thesensed signals in memory 70. Control module 60 may analyze the EGMwaveforms and/or marker channel data to deliver pacing pulses based onthe sensed cardiac events, e.g., pacing pulses triggered or inhibitedbased on the detection or lack of detection of intrinsic cardiacactivity. In some instances, control module 60 may also detect cardiacevents, such as tachyarrhythmia, based on the sensed electrical signals.

Therapy module 64 is configured to generate and deliver electricalstimulation therapy to heart 26. Therapy module 64 may include one ormore pulse generators, capacitors, and/or other components capable ofgenerating and/or storing energy to deliver as pacing therapy. Controlmodule 60 may control therapy module 64 to generate electricalstimulation therapy and deliver the generated therapy to heart 26 viaone or more therapy vectors formed by electrodes 16 and 18 and thehousing electrode of leadless IPG 10 according to one or more therapyprograms, which may be stored in memory 70. Control module 60 controlstherapy module 64 to generate electrical stimulation therapy with theamplitudes, pulse widths, timing, and frequencies specified by aselected therapy program.

In some instances, control module 60 may control therapy module 64 todeliver the pacing therapy based on the electrical signals sensed viathe sensing vector between electrode 16 and 18. Thus, control module 60may control therapy module 64 to deliver pacing therapy using pacingmodes such as AAI or VVI in instances in which leadless IPG 10 isutilized or in modes such as AAI, VVI, DDD, DDI, VAT, VDD, or DVI ininstances in which leadless IPG 40 or cardiac stimulation system 50 areutilized. In other instances, control module 60 may control therapymodule 64 to deliver pacing pulses independent of sensing, e.g., usingasynchronous pacing modes such as AOO, VOO, DOO, or other mode with nosensing or with no inhibiting or triggering in response to sensing,e.g., AAO, VVO, or the like. Control module 60 may control therapymodule 64 to further provide pacing that is rate-responsive in additionto any of the modes described above.

Therapy module 64 may generate and deliver pacing pulses with any of anumber of amplitudes, pulse widths, or other characteristic to captureheart 26. For example, the pacing pulses may be monophasic, biphasic, ormulti-phasic (e.g., more than two phases). The pacing thresholds ofheart 26 when delivering pacing pulses from anterior mediastinum 36 maydepend upon a number of factors, including location, type, size,orientation, and/or spacing of the electrodes, physical abnormalities ofheart 26 (e.g., pericardial adhesions or myocardial infarctions), orother factor(s).

The increased distance from electrodes 16 and 18 to the heart tissue mayresult in heart 26 having increased pacing thresholds compared totransvenous pacing thresholds. To this end, therapy module 64 may beconfigured to generate and deliver pacing pulses having largeramplitudes and/or pulse widths than conventionally required to obtaincapture via transvenously implanted lead or a lead attached to heart 26.In one example, therapy module 64 may generate and deliver pacing pulseshaving amplitudes of less than or equal to 8 volts and pulse widthsbetween 0.5-3.0 milliseconds. In another example, therapy module 64 maygenerate and deliver pacing pulses having amplitudes of between 5 and 10volts and pulse widths between approximately 3.0 milliseconds and 10.0milliseconds. In another example, pulse widths of the pacing pulses maybe between approximately 2.0 milliseconds and 8.0 milliseconds. In afurther example, therapy module 64 may generate and deliver pacingpluses having pulse widths between approximately 0.5 milliseconds and20.0 milliseconds. In another example, therapy module 64 may generateand deliver pacing pluses having pulse widths between approximately 1.5milliseconds and 20.0 milliseconds.

In some cases, therapy module 64 may generate pacing pulses havinglonger pulse durations than conventional transvenous pacing pulses toachieve lower energy consumption. For example, therapy module 64 may beconfigured to generate and deliver pacing pulses having pulse widths ordurations of greater than two (2) milliseconds. In another example,therapy module 64 may be configured to generate and deliver pacingpulses having pulse widths or durations of between greater than two (2)milliseconds and less than or equal to three (3) milliseconds. Inanother example, therapy module 64 may be configured to generate anddeliver pacing pulses having pulse widths or durations of greater thanor equal to three (3) milliseconds. In another example, therapy module64 may be configured to generate and deliver pacing pulses having pulsewidths or durations of greater than or equal to five (5) milliseconds.In another example, therapy module 64 may be configured to generate anddeliver pacing pulses having pulse widths or durations of greater thanor equal to ten (10) milliseconds. In a further example, therapy module64 may be configured to generate and deliver pacing pulses having pulsewidths between approximately 3-10 milliseconds. In a further example,therapy module 64 may be configured to generate and deliver pacingpulses having pulse widths or durations of greater than or equal tofifteen (15) milliseconds. In yet another example, therapy module 64 maybe configured to generate and deliver pacing pulses having pulse widthsor durations of greater than or equal to twenty (20) milliseconds.

Depending on the pulse widths, therapy module 64 may be configured togenerate and deliver pacing pulses having pulse amplitudes less than orequal to twenty (20) volts, deliver pacing pulses having pulseamplitudes less than or equal to ten (10) volts, deliver pacing pulseshaving pulse amplitudes less than or equal to five (5) volts, deliverpacing pulses having pulse amplitudes less than or equal to two andone-half (2.5) volts, deliver pacing pulses having pulse amplitudes lessthan or equal to one (1) volt. In other examples, the pacing pulseamplitudes may be greater than 20 volts. These pulse amplitudes may becombined with any of the pulse widths/durations described above.Reducing the amplitude of pacing pulses delivered by leadless IPG 10 mayreduce the likelihood of extra-cardiac stimulation. Some experimentalresults are provided later illustrating some example combinations ofpacing amplitudes and widths obtained using pacing leads.

Communication module 68 includes any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice, such as a clinician programmer, a patient monitoring device, orthe like. For example, communication module 68 may include appropriatemodulation, demodulation, frequency conversion, filtering, and amplifiercomponents for transmission and reception of data with the aid ofantenna 72. Communication module 68 may communicate with an externaldevice, e.g., an external programmer, to obtain operating parameters orprovide sensed data for analysis or review. In another example,communication module 68 may be configured to communicate with anotherleadless IPG to coordinate delivery of pacing pulses to the atriumand/or ventricle as described with respect to FIGS. 4. Communicationmodule may communicate using any of a number of techniques includinginductive communication, magnetic communication, electromagneticcommunication (e.g., RF), optical communication, tissue conductancecommunication, or the like.

The various modules of leadless IPG 10 may include any one or moreprocessors, controllers, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field-programmable gate arrays(FPGAs), or equivalent discrete or integrated circuitry, includinganalog circuitry, digital circuitry, or logic circuitry. Memory 70 mayinclude computer-readable instructions that, when executed by controlmodule 60 or other component of leadless IPG 10, cause one or morecomponents of leadless IPG 10 to perform various functions attributed tothose components in this disclosure. Memory 70 may include any volatile,non-volatile, magnetic, optical, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),static non-volatile RAM (SRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or any other non-transitory computer-readablestorage media.

Experiments

Three acute procedures were performed using pigs, with the animals in adorsal recumbency. An incision was made near the xiphoid process and aModel 4194 lead was delivered to the substernal/retrosternal space usinga 6996T tunneling tool and sheath. An active can emulator (ACE) wasplaced in a subcutaneous pocket on either the right chest (first acuteexperiment) or the left midaxillary (second and third acuteexperiments). Various pacing configurations were tried and differentpieces of equipment were used as the source of stimulation. Multiplepulse widths were used in delivering the pacing pulse. Acrossexperiments, several different substernal/retrosternal lead electrodelocations were utilized.

In the second and third experiments the impact of lead location onelectrical performance was investigated by moving the lead to severallocations under the sternum and collecting data to generatestrength-duration curves at each location.

In all three acute experiments, the substernal/retrosternal lead wasplaced and electrical data collected. The lead was moved intentionallymany times across experiments to better understand the location bestsuited to capturing the heart at low pacing thresholds, with differentlocations and parameters tried until pacing capability was gained andlost. A range of thresholds based on location and pacing configurationwas recorded. For this reason, the lowest threshold result for eachacute experiment is reported, as are strength-duration curves showingthe range of pacing values obtained from suitable pacing locations. Inall cases, it was determined that positioning thesubsternal/retrosternal pacing electrode approximately over theventricle of the cardiac silhouette provided best results.

Experiment 1

In the first acute study, a MEDTRONIC ATTAIN bipolar OTW 4194 lead(referred to herein as “the 4194 lead”)was implanted substernally, andtwo active can emulators were positioned, one in the right dorsallateral region (ACE1) and one on the right midaxillary (ACE2). The 4194lead was placed directly below the sternum, in the mediastinum, with thelead tip and body running parallel to the length of the sternum. Variouspacing configurations were tried and electrical data collected.

The smallest threshold observed was 0.8 volts, obtained when pacing fromthe tip of the substernal/retrosternal 4194 lead to ACE1 (10 ms pulsewidth and Frederick Heir instrument as the source of stimulation). Itwas possible to capture using a smaller pulse width, though thresholdincreased as the pulse width shortened (1.5V at 2 ms in this sameconfiguration with the Frederick Heir Stimulator. Many additional lowthresholds (1-2 volts) were obtained with different pacingconfigurations and pulse durations.

FIG. 6 illustrates a strength-duration curve showing the capturethresholds obtained at various pulse widths during the first acutestudy. Note that all configurations paced from either the tip or thering of the substernally implanted 4194 lead (−) to one of the twoactive can emulators (+). In one instance, a large spade electrode(instead of a Model 4194 lead) was used as the substernal/retrosternalelectrode, as noted in the legend of.

As shown, several pacing configurations and parameters were tried.Across the configurations reported in the graph above, threshold valuesranged from 0.8 volts to 5.0 volts, with threshold generally increasingas pulse width was shortened. In a few instances, the threshold at 1.5ms pulse width was smaller than the threshold at 2.0 ms. It should benoted that the threshold value obtained at 1.5ms was always recordedusing the Medtronic 2290 analyzer as the stimulation source, wheras allother threshold measurements for the first acute experiment (at pulsewidths of 2, 10, 15 and 20 ms) were obtained using a Frederick Heirinstrument as the source of stimulation. Differences in these twoinstruments may account for the difference in threshold values atsimilar pulse widths (1.5 ms and 2 ms).

In general, the first acute experiment demonstrated the feasibility ofsubsternal/retrosternal pacing by producing small capture thresholds(average=2.5±1.2 volts), using several different pacing configurationsand parameters.

Experiment 2

A second acute experiment was conducted. In the second acute, however,the animal presented with pericardial adhesions to the sternum. Becauseof the pericardial adhesion, the ventricular surface of the cardiacsilhouette was rotated away from the sternum—an anatomical differencethat may have resulted in higher thresholds throughout this experiment.

As in the previous acute experiment, a Model 4194 lead was placed underthe sternum. An active can emulator was placed on the left midaxillary.The tip to ring section of the 4194 was positioned over the cardiacsilhouette of the ventricle, as observed by fluoroscopy, and thisposition is notated “Position A” on the strength-duration graphillustrated in FIG. 7. The lead eventually migrated a very shortdistance closer to the xiphoid process during stimulation (still underthe sternum) to reach “Position B,” and additional electricalmeasurements were obtained successfully from this position as well.

The smallest threshold observed in the second acute experiment was 7V,obtained when pacing from the substernal/retrosternal 4194 ringelectrode (−) to an ACE (+) on the left midaxillary in the first leadposition (5 ms, 15 ms and 20 ms pulse widths, Frederick Heirstimulator). Additionally, thresholds of 8 and 9 volts were obtainedwith the lead in the second anatomical position, both from 4194 tip toACE (unipolar) and 4194 tip to ring (bipolar) configurations at multiplepulse widths. The two lines that appear to run off the chart wereinstances of no capture.

All of the electrical values reported in FIG. 7 were collected with theFrederick Heir instrument as the stimulation source. Extra-cardiacstimulation was observed with many of the electrical measurementsobtained in a unipolar pacing configuration. No obvious extra-cardiacstimulation was observed when pacing in a bipolar configuration (4194tip to ring), though a low level of stimulation could be felt with ahand on the animal's chest.

Experiment 3

A third and final acute experiment was conducted demonstrating thefeasibility of substernal/retrosternal pacing. As in the previous twoacute experiments, a 4194 lead was placed under the sternum. An activecan emulator was placed on the left midaxillary. In this experiment, thesubsternal/retrosternal 4194 lead was intentionally positioned so thatthe lead tip was initially near the second rib, far above the cardiacsilhouette of the ventricle. The lead tip was then pulled back (towardthe xiphoid process) one rib space at a time, collecting electrical dataat each position. As in previous experiments, low capture thresholdswere obtained when the pacing electrodes were approximately positionedover the ventricular surface of the cardiac silhouette, as observed viafluoroscopy. When the lead tip was not over the ventricular surface ofthe cardiac silhouette, “no capture” was often the result.

As in previous experiments, pacing was performed from either the tip orthe ring of the substernal/retrosternal 4194 lead (−) to the ACE (+) onthe left midaxillary. However, in this acute experiment, a subcutaneousICD lead was also positioned in its subcutaneous arrangement (asillustrated and described in FIGS. 1A-C). In some instances, the pacingconfiguration was from either the tip or the ring of thesubsternal/retrosternal 4194 lead (−) to either the ring or the coil ofthe subcutaneous ICD lead (+), so that the ICD lead and not the ACE wasthe indifferent electrode.

The smallest threshold observed across the experiment was 0.8V, obtainedwhen pacing from the substernal/retrosternal 4194 tip electrode (−) toan ACE (+) on the left midaxillary when the lead was positioned suchthat the lead tip electrode was approximately under the sixth rib (20 mspulse width and Frederick Heir stimulator). Many additional lowthresholds were obtained with different pacing configurations, shorterpulse durations and different lead positions, again demonstrating thefeasibility of substernal/retrosternal pacing. Obvious extra-cardiacstimulation generally was not observed with lower threshold measurements(at longer pulse durations) but was observed at higher thresholds.

The strength duration curves for lead positions 3-5 are presented inFIGS. 5-7, with individual graphs for each location due to the breadthof electrical data collected. Measurements made with the 2290 analyzeras the source of stimulation are noted. Other electrical measurementswere made with the Frederick Heir instrument as the stimulation source.

FIG. 8 illustrates the strength-duration curve of electrical data fromthe third acute experiment when the 4194 lead tip was positioned underthe sternum at the location of the 4^(th) rib. Several therapy vectorsresulted in low pacing thresholds, generally when pulse widths werequite long. At shorter pulse widths, threshold increased.

FIG. 9 illustrates the strength-duration curve of electrical data fromthe third acute experiment when the 4194 lead tip was positioned underthe sternum at the location of the 5^(th) rib. The two lines that appearto run off the chart at 0.2 ms were instances of no capture. FIG. 9demonstrates the position dependence of the substernal/retrosternallead. Thresholds were higher overall in this anatomical location (thelead tip near the 5^(th) rib), though capture was still possible and inthe 4194 ring (−) to ACE (+) configuration, moderately low (2 volts at20 ms). There generally was no significant extra-cardiac stimulationobserved except with pulse widths of 0.2 ms and 0.5 ms in the 4194 tip(−) to ACE (+) configuration and in the unipolar configuration goingfrom the 4194 tip (−) to the coil of the subcutaneous ICD lead at pulsewidths of 1.5 ms and shorter, all of which resulted in the highestrecorded threshold readings in this lead position.

FIG. 10 illustrates the strength-duration curve of electrical data fromthe third acute experiment when the 4194 lead tip was positioned underthe sternum at the location of the 6^(th) rib. FIG. 10 shows theposition dependence of the substernal/retrosternal electrode. When thepacing electrode is optimally located over the ventricular surface ofthe cardiac silhouette (as observed via fluoroscopy), pacing thresholdis low. Low thresholds were very repeatable in this anatomical location,even at shorter pulse durations and in many different pacingconfigurations. Extra-cardiac stimulation generally was not apparent atlow thresholds and longer pulse durations throughout this experiment.

All three acute experiments demonstrated the feasibility of pacing froma substernal/retrosternal electrode location. The lowest thresholdresults across the three acute procedures were 0.8 volts, 7 volts and0.8 volts, respectively, with the second acute procedure involving ananatomical difference (pericardial adhesions) that tipped theventricular surface of the heart away from its normal orientation withthe sternum, resulting in higher pacing thresholds. However, for thepurposes of anti-tachycardia pacing, conventional devices typicallydefault to maximum output (8V at 1.5 ms) for ATP therapy delivery. Giventhis, even the 7V threshold obtained in the second acute experimentcould be satisfactory for ATP therapy.

The ability to capture the heart at low pacing thresholds was dependentupon electrode position. As observed through these experiments, thesubsternal/retrosternal pacing electrode provides the best outcomes whenpositioned approximately over the ventricular surface of the cardiacsilhouette, which is easily observed via fluoroscopy and encompasses areasonably large target area for lead placement. In the third acuteexperiment, for example, capture was achieved at three separatepositions, with the lead tip at approximately ribs 4, 5 and 6, all ofwhich were near the ventricular surface of the cardiac silhouette.

Pacing thresholds increased with shorter pulse durations. In manyinstances, however, low pacing thresholds were obtained even at shortpulse widths, especially when the substernal/retrosternal pacingelectrode was positioned over the ventricular surface of the cardiacsilhouette. In other instances, longer pulse durations (10-20 ms) werenecessary to obtain capture or to achieve lower capture thresholds.

Across experiments, it was possible to pace from thesubsternal/retrosternal lead to an active can emulator positioned nearthe animal's side (unipolar) and also from the substernal/retrosternallead to a subcutaneous ICD lead (unipolar). If a subcutaneous ICD systemincorporated a lead, placed substernally, for the purpose ofanti-tachycardia pacing, both of the aforementioned unipolar pacingconfigurations would be available for a physician to choose from.

These experiments also demonstrated the ability to pace in a bipolarconfiguration entirely under the sternum (4194 tip (−) to 4194 ring (+),substernally), indicating that either a bipolar lead positioned underthe sternum might be used for anti-tachycardia pacing purposes.

Overall, the results of these acute experiments demonstrate the abilityto pace the heart from a substernal/retrosternal location, with the leadnot entering the vasculature or the pericardial space, nor makingintimate contact with the heart. The low threshold values obtained whenpacing from a substernal/retrosternal lead location in these acuteexperiments suggest that pain-free pacing for the purpose ofanti-tachycardia pacing in a subcutaneous ICD system is within reach.

In some instances, the electrodes of the various leadless IPGs describedherein may be shaped, oriented, designed or otherwise configured toreduce extra-cardiac stimulation. For example, the electrodes of thevarious leadless IPGs described herein may be shaped, oriented, designedor otherwise configured to focus, direct or point the electrodes towardheart 26. In this manner, pacing pulses delivered via the electrodes ofthe various leadless IPGs described herein are directed toward heart 26and not outward toward skeletal muscle. For example, the electrodes ofthe various leadless IPGs described herein may be partially coated ormasked with a polymer (e.g., polyurethane) or another coating material(e.g., tantalum pentoxide) on one side or in different regions so as todirect the pacing signal toward heart 26 and not outward toward skeletalmuscle.

Various examples have been described. These and other examples arewithin the scope of the following claims.

1. An implantable cardiac pacing system comprising: a housing; a firstelectrode on the housing; a second electrode on the housing; and a pulsegenerator within the housing and electrically coupled to the firstelectrode and the second electrode, wherein the housing is implantedsubstantially within an anterior mediastinum of a patient and the pulsegenerator is configured to deliver pacing pulses to a heart of thepatient via a therapy vector formed between the first and secondelectrodes.
 2. The system of claim 1, wherein the pulse generator isconfigured to provide one of bradycardia pacing, antitachycardia (ATP)pacing, and post-shock pacing to the patient via the lead.
 3. The systemof claim 1, wherein the housing is implanted substantially within theanterior mediastinum such that the first and second electrodes arelocated over one of a cardiac silhouette of a ventricle of the heart asobserved via an anterior-posterior (AP) fluoroscopic view of the heart.4. The system of claim 3, further comprising: a third electrode on thehousing; and a fourth electrode on the housing, wherein the housing isimplanted substantially within the anterior mediastinum such that thethird and fourth electrodes are located over a cardiac silhouette of anatrium of the heart as observed via the AP fluoroscopic view of theheart.
 5. The system of claim 1, further comprising: a third electrodeon the housing; and a fourth electrode on the housing, wherein thehousing is implanted such that the pulse generator provides pacingpulses to a ventricle of the heart via the first and second electrodeand provides pacing pulses to an atrium of the heart via the third andfourth electrodes.
 6. The system of claim 1, wherein the housingcomprises a first housing and the pulse generator comprises a firstpulse generator, the system further comprising: a second housingseparate from the first housing; a third electrode on the secondhousing; a fourth electrode on the second housing; and a second pulsegenerator within the second housing, wherein the first and secondhousings are implanted substantially within the anterior mediastinum ofthe patient such that pacing pulses are delivered to a first chamber ofthe heart via the first and second electrodes of the first housing andpacing pulses are delivered to a second chamber of the heart via thethird and fourth electrodes of the second housing.
 7. The system ofclaim 6, further comprising: a first communication module within thefirst housing; and a second communication module within the secondhousing, wherein the first communication module transmits a telemetrycommunication to the second communication module to coordinate thepacing pulses delivered to the first and second chambers of the heart.8. The system of claim 1, wherein the housing comprises a first housingand the pulse generator comprises a first pulse generator, the systemfurther comprising: a second housing separate from the first housing; athird electrode on the second housing; a fourth electrode on the secondhousing; and wherein the first and second housings are implantedsubstantially within the anterior mediastinum of the patient such thatthe first and second electrodes of the first housing sense electricalsignals of a first chamber of the heart and the third and fourthelectrodes of the second housing sense electrical signals of a secondchamber of the heart.
 9. The system of claim 1, wherein the pulsegenerator is configured to deliver pacing pulses having pulse widthsgreater than or equal to two (2) milliseconds.
 10. The system of claim1, wherein the pulse generator is configured to deliver pacing pulseshaving pulse widths between approximately one and a half (1.5)milliseconds and twenty (20) milliseconds.
 11. The system of claim 1,wherein the pulse generator is configured to deliver pacing pulseshaving pulse widths greater than two (2) milliseconds and less thaneight (8) milliseconds.
 12. The system of claim 1, wherein the pulsegenerator is configured to deliver pacing pulses having pulse amplitudesbetween approximately one (1) and twenty (20) volts.
 13. A methodcomprising: providing a leadless implantable pulse generator (IPG)substantially within an anterior mediastinum of a patient, the leadlessIPG including a housing, first electrode on the housing, a secondelectrode on the housing, and an pulse generator within the housing andelectrically coupled to the first electrode and the second electrode;generating one or more stimulation pulses with the implantable pulsegenerator; and delivering the one or more stimulation pulses to a heartof the patient via the first and second electrode of the leadless IPGimplanted substantially within the anterior mediastinum.
 14. The methodof claim 13, wherein delivering the one or more stimulation pulsescomprises delivering one or more bradycardia pacing pulses,antitachycardia (ATP) pulses, and post-shock pacing pulses.
 15. Themethod of claim 13, wherein the leadless IPG further includes a thirdelectrode on the housing and a fourth electrode on the housing anddelivering the one or more stimulation pulses comprises deliveringstimulation pulses to a ventricle of the heart via the first and secondelectrodes on the housing and delivering one or more stimulation pulsesto an atrium of the heart via the third and fourth electrodes on thehousing.
 16. The method of claim 15, wherein the leadless IPG isimplanted substantially within the anterior mediastinum such that thefirst and second electrodes are located over a cardiac silhouette of aventricle of the heart as observed via an anterior-posterior (AP)fluoroscopic view of the heart and the third and fourth electrodes arelocated over a cardiac silhouette of an atrium of the heart as observedvia the AP fluoroscopic view of the heart.
 17. The method of claim 13,wherein the leadless IPG comprises a first leadless IPG, the methodfurther comprising: providing a second leadless implantable pulsegenerator (IPG) substantially within the anterior mediastinum of thepatient, the second leadless IPG including a second housing, thirdelectrode on the second housing, a fourth electrode on the secondhousing, and a second pulse generator within the housing andelectrically coupled to the third electrode and the fourth electrode;generating one or more stimulation pulses with the second implantablepulse generator; and delivering the one or more stimulation pulses, viathe third and fourth electrode of the second leadless IPG, to adifferent chamber of the heart than the stimulation pulses delivered bythe first leadless IPG.
 18. The method of claim 17, further comprisingtransmitting a communication from the first leadless IPG to the secondleadless IPG to coordinate the stimulation pulses delivered to thedifferent chambers of the heart.
 19. The method of claim 13, whereingenerating and delivering the one or more stimulation pulses comprisesgenerating and delivering one or more stimulation pulses having pulsewidths between approximately one and a half (1.5) milliseconds andtwenty (20) milliseconds.
 20. The method of claim 13, wherein generatingand delivering the one or more stimulation pulses comprises generatingand delivering one or more stimulation pulses having pulse widthsgreater than two (2) milliseconds and less than eight (8) milliseconds.